1. Technical Field of the Invention
The present invention relates to an apparatus for driving a sensorless motor such as a brushless DC motor or a stepper motor, and more particularly, to an apparatus for driving a sensorless motor by controlling commutation by means of a position detector disposed on an object driven by the sensorless motor.
2. Description of the Related Art
In the prior art of motors using a permanent magnet on a rotor, such as a brushless DC motor or an HB-type stepper motor, it is known to detect the position of the rotor by detecting a counter electromotive force induced in a non-exciting phase (phase in which no current is passed) of stator coils.
That is, the counter electromotive voltage induced in a non-exciting coil is monitored, and the position of the rotor is detected by detecting a zero-crossing point at which the counter electromotive voltage crosses a neutral voltage point. In this technique, commutation is performed, for example, at a point at which the phase is shifted by 30xc2x0 with respect to the zero-crossing point.
However, in this technique, when the rotor is at rest, no counter electromotive voltage is induced in the stator coil and thus sensorless driving is impossible. Therefore, when the motor whose rotor is at rest is started, the rotor is forcedly driven by performing forced commutation, and the operation is switched into a sensorless driving mode when the rotation speed has become high enough to induce a counter electromotive voltage greater than a predetermined value in the stator coil.
It is also known to provide a Hall device on a motor whereby the position of the rotor is detected thereby controlling the motor in a sensorless control method.
However, in the sensorless control on the basis of the counter electromotive voltage, commutation cannot be controlled in a low-speed range as described above, and thus this technique is unsuitable when the motor is frequently started and stopped. On the other hand, when a Hall device is used to control commutation, although control is possible in a low-speed range, the control accuracy is limited by factors such as a limited accuracy in the width of magnetic poles of the rotor and an installation position error of the Hall device, and thus the accuracy of controlling commutation is poorer than that obtained by the sensorless control on the basis of the counter electromotive voltage in a high-speed range in which the counter electromotive voltage can be detected. Thus, there is a need for a sensorless control method which allows a motor to be controlled precisely even in a low-speed range.
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an apparatus for driving a sensorless motor, capable of controlling commutation in a precise and highly reliable fashion even in a low-speed range.
To achieve the above object, there is provided an apparatus for driving a sensorless motor, comprising: a position detector for outputting a pulse signal in response to movement of an object driven by a sensorless motor; commutation control means which counts the number of pulses output from the position detector and controls the commutation of the sensorless motor depending upon the counted value; and commutation reference point setting means for setting a commutation reference point employed as a reference point in the counting of the pulses; wherein, when the sensorless motor is started for the first time, the commutation reference point setting means performs phase excitation twice, switching the excited phase such that a pulled-in position resulting from the first-time excitation and that resulting from second-time excitation become different in electrical angle from each other by a magnitude not equal to either 180xc2x0 or an integral multiple of 180xc2x0, and the commutation reference point setting means performs the setting of the commutation reference point when a rotor of the motor has stopped after the second-time excitation.
When the object driven by the sensorless motor moves, a pulse signal is output from the position detector in response to the movement of the object, and commutation is controlled on the basis of the counted number of pulses of the pulse signal. The commutation reference point, which is used as a reference point in counting the number of pulse signals, is set by the commutation reference point setting means on the basis of a position (pulled-in position) at which the rotor stops when a stator coil of the sensorless motor is excited for the first time.
In the case where excitation is performed only once, there is a possibility that, depending upon the position at which the rotor is at rest just before the sensorless motor is started for the first time, the rotor does not move in response to the excitation. However, if excitation is performed twice such that pulled-in positions become different in electrical angle from each other by a magnitude not equal to either 180xc2x0 or an integral multiple of 180xc2x0, it is assured that the rotor moves. If the commutation reference point is set at the position at which the rotor stops after being pulled therein, the resultant commutation reference point becomes coincident with the position at which commutation should be performed during the rotation of the rotor.
Therefore, if the number of pulses is counted starting from the commutation reference point, and if commutation is performed each time the counted value becomes equal to an integral multiple of the predetermined number of pulses per commutation interval, commutation is correctly performed whenever the rotor comes to a position at which commutation should be performed.
The invention also provides a sensorless motor driving apparatus wherein the commutation reference point setting means sets the commutation reference point for each rotation direction of the sensorless motor, and wherein the commutation control means controls the commutation depending upon a present rotation direction in accordance with the number of pulses as counted starting from a commutation reference point set for the present rotation direction.
The invention also provides a sensorless motor driving apparatus wherein the commutation reference point setting means detects an offset value indicating the number of pulses corresponding to a difference in position between the commutation reference points set for the respective rotation directions of the sensorless motor, and wherein the commutation control means counts the pulses with respect to one of commutation reference points and corrects the counted value of pulses on the basis of the offset value each time the rotation direction is switched.
Commutation reference points are set for the respective rotation directions of the sensorless motor. In the case where the rotational motion of the sensorless motor is converted into the linear motion of a belt via a pulley thereby moving an object disposed on the belt, the absolute position of the rotor corresponding to a certain absolute position of the object becomes different depending upon the rotation direction, because the amount of expansion of the belt varies depending upon the rotation direction. When there is such a difference, if commutation is controlled on the basis of the counted value of pulses with reference to one commutation reference point set for one rotation direction, commutation timings determined on the basis of the counted value of pulses in the opposite direction become different from correct timings.
To avoid the above problem, commutation reference points are set for the respective rotation directions of the sensorless motor, and commutation is controlled, depending upon the present rotation direction, in accordance with the number of pulses as counted starting from the commutation reference point set for the present rotation direction, thereby preventing the commutation timings from shifting from the correct timings.
Herein, an offset value, which indicates the number of pulses corresponding to a difference in position between the commutation reference points set for the respective rotation directions of the sensorless motor, is detected, and the number of pulses is counted with respect to one of commutation reference points and the counted value of pulses is corrected on the basis of the offset value each time the rotation direction is switched so that the resultant number of pulses represents a correct number with respect to the commutation reference point for the corresponding rotation direction. In this case, it is not necessary to use two counting variables for the respective rotation directions.
The invention also provides a sensorless motor driving apparatus further comprising counter electromotive force detection means for detecting a counter electromotive force induced in a non-exciting phase of the sensorless motor; and commutation timing generating means for generating a commutation timing in accordance with the counter electromotive force detected by the counter electromotive force detection means, wherein commutation reference point setting means resets the commutation reference point at a point of time of the commutation timing generated by the commutation timing generating means.
The invention also provides a sensorless motor driving apparatus wherein the commutation timing generating means generates a commutation timing in accordance with the counter electromotive force of one of phases of the sensorless motor.
The counter electromotive force induced in the non-exciting phase, in which no current is passed, of the sensorless motor is detected by the counter electromotive force detection means. The position of the rotor is detected on the basis of the detected counter electromotive force, and a commutation timing is generated on the basis of the detected position of the rotor. At a point of time indicated by the generated commutation timing, resetting of the commutation reference point is performed.
The accuracy of the position of the rotor detected on the basis of the counter electromotive force is better than the accuracy of the position of the rotor detected on the basis of the pulled-in position where the rotor is pulled in by pulling-in excitation. That is, in the case where the rotor is pulled into a rest position by excitation, the rotor stops at a position slightly shifted from an electrical stable rest position, depending upon an external force such as a frictional force which is balanced with a torque generated by the motor. In contrast, the position of the rotor detected by the counter electromotive force does not include an error caused by the frictional force or the like.
In view of the above, when it becomes possible to generate a commutation timing on the basis of the counter electromotive force, the position of the rotor is detected more precisely on the basis of the counter electromotive force, and a commutation timing is generated on the basis of the detected position of the rotor, and resetting of commutation reference point is performed. After the resetting of the commutation reference point, the number of pulses is counted with respect to the commutation reference point, and commutation is performed on the basis of the counted value of pulses, and thus it is ensured that commutation is controlled more precisely.
In the conventional sensorless control technique on the basis of the counter electromotive force, it is necessary to sequentially detect counter electromotive forces of all phases and generate commutation timings for all phases. In contrast, the commutation timing generating means generates commutation timings on the basis of the counter electromotive force not of all phases of the sensorless motor but of one of the phases, and thus the counter electromotive force can be detected using a fewer number of circuits, and the commutation timing can be generated by simpler processing.
The invention also provides a sensorless motor driving apparatus wherein the commutation timing generating means generates a commutation timing for each rotation direction of the sensorless motor, and wherein the commutation reference point setting means resets the commutation reference points for the respective rotation directions in accordance with the corresponding commutation timings generated for the respective rotation directions.
The invention also provides a sensorless motor driving apparatus wherein the commutation reference point setting means detects an offset value indicating the number of pulses corresponding to a difference in position between the commutation reference points reset in accordance with the commutation timings generated for the respective rotation directions by the commutation timing generating means, and the commutation reference point setting means includes storage means for storing the offset value, and wherein the commutation control means counts pulses with respect to one of the reset commutation reference points and corrects the counted value of pulses on the basis of the offset value stored in the storage means each time the rotation direction is switched.
The commutation reference point setting means resets the commutation reference points on the basis of the commutation timings generated for the respective rotation directions by the commutation timing generating means. In the case where the rotational motion of the sensorless motor is converted into the linear motion of a belt via a pulley thereby moving an object disposed on the belt, the absolute position of the rotor corresponding to a certain absolute position of the object becomes different depending upon the rotation direction, because the amount of expansion of the belt varies depending upon the rotation direction. When there is such a difference, if commutation is controlled on the basis of the counted value of pulses with reference to one commutation reference point set for one rotation direction, commutation timings determined on the basis of the counted value of pulses in the opposite direction become different from correct timings.
To avoid the above problem, commutation timings are generated on the basis of the counter electromotive forces in the respective rotation directions, and resetting of the commutation reference points is performed for the respective rotation directions on the basis of the commutation timings generated, thereby preventing the commutation timings from shifting from the correct timings.
Herein, an offset value is detected which indicates the number of pulses corresponding to a difference in position between the commutation reference points set for the respective rotation directions of the sensorless motor, and the detected offset value is stored in the storage means. After resetting the commutation reference points, the number of pulses is counted with respect to one of the commutation reference points, and the counted value of pulses is corrected on the basis of the offset value stored in the storage means each time the rotation direction is switched, so that the corrected counted value represents the number with respect to the commutation reference point corresponding to the present rotation direction. In this case, it is not necessary to use two counting variables for the respective rotation directions.
The invention also provides a sensorless motor driving apparatus wherein when controlling of the sensorless motor is started, the commutation control means performs the resetting of a commutation reference point in accordance with a commutation timing generated by the commutation timing generating means.
The invention also provides a sensorless motor driving apparatus wherein when a predetermined period of time has elapsed since controlling of the sensorless motor was started, the commutation control means performs the resetting of a commutation reference point in accordance with a commutation timing generated by the commutation timing generating means.
The invention also provides a sensorless motor driving apparatus wherein each time a predetermined period of time elapses after controlling of the sensorless motor was started, the commutation control means performs the resetting of a commutation reference point in accordance with a commutation timing generated by the commutation timing generating means.
The invention also provides a sensorless motor driving apparatus wherein each time the sensorless motor is started, the commutation control means performs the resetting of a commutation reference point in accordance with a commutation timing generated by the commutation timing generating means.
The resetting of the commutation reference point on the basis of the commutation timing generated by the commutation timing generating means is performed when the control of the sensorless motor is started, and the commutation reference point is reset as soon as the rotation speed of the sensorless motor becomes high enough to generate a commutation timing on the basis of counter electromotive force. Therefore, the commutation reference point can be set precisely in early stage after starting the control of the sensorless motor.
If the resetting of the commutation reference point is performed when a predetermined period of time has elapsed since the control was started, more specifically, for example, if the resetting is performed when a change in the ambient temperature, which occurs after the sensorless motor is started, has reached an equilibrium state, the resetting of the commutation reference point is performed under a stable condition in terms of the ambient temperature. Furthermore, if the resetting of the commutation reference point is performed each time a predetermined period of time elapses after the control is started, the commutation reference point is properly set depending upon a change in the ambient temperature. Still furthermore, if the resetting of the commutation reference point is performed each time the sensorless motor is started, that is, each time the sensorless motor starts to rotate, the commutation reference point is correctly set depending upon the conditions when the sensorless motor is started.